Induction heating method for making molds



United States Patent 3,259,947 INDUCTION HEATINC3$IHOD FOR MAKING MHarry R. Knight, 1535 Oregon Ave., Muskegon, Mich. No Drawing. FiledNov. 5, 1964, Ser. No. 409,314 3 Claims. (Cl. 22-193) This inventionrelates to articles formed of particulate refractory materials boundtogether by a binder into a selected configuration, particularly moldand core type articles for molding. This invention relates morespecifically to a method of and a composition for forming cores andmolds and other configurated articles of refractory materials boundtogether by using heat.

The art of forming molds and cores for metal casting and moldingprocesses necessitates interbinding of refractory particles, e.g., sand,into a self-sustained mold or core with a specific innersurfaceconfiguration. Prior to the late l940s, this was done primarily withsand molds, using an oil type binder. This older method is presentlysupplemented largely by shell molding techniques, using a resin binder.

The binders of very simple sand cores, or shell molds or cores, is curedby baking articles with heat in a large controlled oven. However, whenthe mold or core configuration is complex, especially when it hasportions of differing thickness, uniform curing becomes a major problemand cannot be obtained with an oven. In fact, for most complex core worktoday, complicated and extremely expensive core making boxes are firstcarefully made, each with an intricate system of manifolds and aplurality of gas burners, to try to obtain proper heat distributionthrough the core box to the core material for uniform baking. Sincethese cores assume a variety of configurations, and must be made toexact tolerances, each metal box is an intricate work of metal formingand finishing, carefully done by highly paid craftsmen from expensivematerials that will conduct heat and yet resist breakdown under heat.This all causes a very great expense, as is well-known.

Frequently, the burner arrangement for core boxes must be re-vamped onrepeated trial and error methods after initial formation of the corebox, to obtain the exact amount of heat necessary for each thin, thickand intermediate section of varying configuration. Very often, initialburner arrangements have to be changed by moving individual burners,adding certain burners, plugging burner mounting holes, and so forth.

Differences in the bulk of various sections of the core itself createproblems of heat transmission through the section. Ideally, forcompletely uniform curing, the heat in the center of the section shouldbe the same as that at the outer surface where the heat is applied. Thisis never really the situation of course, due to the fact that the heatis transmitted indirecly through the core box itself, and then throughthe core from its exterior to its interior.

Even when these factors have been taken into consideration and the corebox is specially engineered and revamped to suit a particular core,subsequent erosion, warpage, and corrosion of the box occurs constantlyalong the carefully formed surfaces and on the precise drawing andguiding mechanisms, due to the high temperatures to which they areexposed. This, of course, seriously affects the quality of the products.

Further, vent systems and blow systems operate at a low efiiciency underthese high heat conditions, sometimes even making it necessary toinstall complicated local coolant arrangements at various points in theheating core box.

Also, with the use of these core boxes, installation of the equipmentrequires expensive custom designing of against the next set of shells.

the core blowing machine itself for each different installation.

During the use of hot core box equipment, the excessive heat lossoccurring into the foundry, coupled with the resulting smoke and fumes,create a definite working condition problem. Further, loss of heat dueto shift changes, mechanical failure, etc. requires considerable downtime to reheat the core box equipment whenever this occurs.

Briefly, therefore, these hot core box techniques, while reasonableserving the purpose to form intricately shaped cores, present tremendousexpenses and problems in their manufacturing and use, and still providea core that could be substantially improved since it is not reallyuniformly cured throughout.

The shell molding process has another substantial disadvantagetherequirement of heavy backing material, such as sand or shot, to supportthe shell elements in their face-to-face contact. Shell molds are madewith uniform thickness to enable quite uniform curing of the shells inan oven. In this respect, the problems encountered with curing thedifferent thickness sections of a core are not normally found to aconsiderable extent in shell molds, because of their uniform thickness.However, the uniform thickness of the shell does require this resultingsupport disadvantage causing bulky, large assemblies for each pair ofshells. The shot or sand material must be removed after each molding andsubsequently replaced This is time consuming, space consuming and hence,adds a substantial cost to the system.

With these factors in mind, it is an object of this invention to providea novel core and mold composition, and a novel method of curing moldand/or core materials to actually enable the complete elimination ofexpensive, elaborate hot core boxes and all of the associated equipment.The vast expense involved with the intricate forming of the accuratemetal surfaces, providing the complex manifolds, and installing thelarge number of specially placed burners, as Well as the resultingcorrosion, Warpage and handling problems, are all made completelyunnecessary. The resultant savings in time, labor, and costs and theresulting advancement in efficiency of operations, space, personnel timeis tremendous. The capacity for forming and uniformly curing exceedinglydelicate and/or complex cores, molds, and the like is increasedmanifold.

Another object of this invention is to provide a novel method of curingcores, molds and other like articles, whereby materials such as non-heatconductive materials including plastic or wood can even be used for themold forming patterns, rather than specially machined heat conductivemetal. Inexpensive plastic or wood patterns can be quickly formed, evenon a mass production basis, to enable dependable core and/ or moldmaking with complete and exact accuracy.

It is another object of this invention to provide a core and moldcomposition, and a method of curing cores and molds that uniquelyapplies the exact amount of heat necessary for each section of eachportion of the article, whether it is thick or thin, whether it iscurved, fiat or of other configurations. Moreover, it applies heat indirect proportion to the mass of that portion, so that all parts areheated completely uniformly, even the center of thick portions. Theresult is a completely uniform curing of the article binder. No portionsare over cured or under cured.

Another object of this invention is to provide a novel composition andmethod for forming articles such as cores or molds, where heat is notapplied externally, but rather is created immediately at all exactinternal zones of application, and in exact proportion to the massinvolved, with each unit mass being able to receive the same heat,whether it is deep in a thick article portion or adjacent the surface.This achieves completely optimum curing in the core and/or mold.Moreover, complete curing .is achieved in an extremely short timeinterval of about one to five seconds, i.e., about one-tenth toonetwentieth of that previously required.

Another object of this invention is to provide a ,novel composition andmethod whereby shell molds can be formed with a structure configuratedso that a series of mold sections will stack together face-to-face andbackto-back, without any reinforcing backing material. The inventionenables them to be made of varying thicknesses so that they will stillhave a specially configurated, accurately formed face surface forcooperation with a like face surface to form the mold cavity, and with aflat back surface for abutment with the like fiat back surface of anadjacent mold section, yet with capacity for completely uniform curingthroughout.

It is a further object of this invention to provide a method of curing abinder material in a form of refractory particles, to form an article ofselected configuration whether the article is of the foundry type suchas molds and cores, or some other item such as ceramic articles of claybase materials for use as the final article, or for use as a ceramicmold, or otherwise.

These and several other objects of this invention will become apparentupon studying the following detailed description.

This invention is especially useful for making cores and molds forfoundry use. Indeed, this is the main area for which the invention wasconceived and tested. Particularly, it has unique application to resinbinder type cores and molds, often termed in the trade as part of theshell molding process," because of the shape of the typical shell moldsections which were originally formed by this method. The invention isalso applicable in its broader aspects to other molding techniques suchas ceramic molding, and sand molding using an oil binder. It .isfurthermore applicable to other types of articles and foundry type moldsand cores, e.g., ceramic articles, or other articles formed of bondedrefractory particles from a loose particulate condition to a finalintegrated state.

Since, however, its main use is intended for molds and.

cores, it will be chiefly described with respect thereto.

When forming molds and cores, a refractory particulate material, i.e.,one resistant to break down under heat, is interbonded from an initialparticulate condition to a final, desired configuration, in aself-sustaining, integral article. The typical refractory materials usedinclude acid types such as silica and fire clay, basic types such asmagnesite and dolomite and neutral types such as chromite and graphite.

When making resin bonded type cores and molds, a heat curable resin isemployed. A great many of such resins are known today. The most popularare formaldehyde,

resins including phenolic, melamine, urea, cresol, and thioureaformaldehyde resins. Typical of these are the phenol formaldehyde resinstaught for example in United States Patents Nos. 2,706,163, 2,718,681,2,706,188, 2,683,296 and 3,140,518. As is well-known to those in theart, the basic components of these comprise the suitable resin such asphenol formaldehyde, often a few additives such as cryolite and alsooften kerosene or core oil as dampeners and cereal as a supplementalcore binder. Several other ingredients may be added further, as is knownin the trade, to obtain particular characteristics. These form nospecial part of my invention.

A typical thermosetting resin used in the trade is known as 10 4576 AcmeResin BondSand. sold .by the Acme Resin Company of Chicago, Illinois.Often the material is slightly varied in composition if it is used as aphenolic 4 shell resin is General Electrics 5-1073 and a typical corebinder is General Electrics 12353. A typical curing agent used with theresin is hexamethylene tetrarnine.

Of course, the basic shell molding techniques were taught in the fieldinformation agency technical bulletin of the United States Department ofCommerce Fiat Final Report No. 1168 of May 1947. The amount of binderoften varies, usually within /212% by weight of the refractory. 7

Typical of the sand and drying oil mixtures used is a silica sand mixedwith a linseed oil, as is well-known to.

those in the foundry field. The amount of linseed oil will varydepending upon the thickness of the mold and core, the use to which itis to be put, the type of metal to be cast, the type of sand, the typeof characteristics desired in the core, and other factors as iswell-known. Various additional additives can be mixed with themoldingsand to achieve desired characteristics. Since all of thesefactors are well-known in the foundry field, and since detailed examplesof all of these materials would require extensive disclosure in aneedless manner, further descriptive material is not given.

The inventive concept involves one particular aspect of the compositionformed from the refractory materials and a heat curable binder. Othermaterials may or may not the employed, it does not matter. In thisrespect, the term heat curable includes, in its broadest aspects, thetypical setting of a core caused by drying of a linseed oil type binder,as well as the chemical curing of a synthetic resin binder. broadestaspects, to encompass the direct fusion of ceramic type materialstogether due to interbinding of the particulate materials.

The invention was discovered in efforts for a solution to the core boxdifliculties, and in efforts to really obtain uniform curing. Theinventor conceived of a new method employing a special additive to therefractory material to achieve just that. Experimentation proved theinventive concept to be completely sound.

The novel composition is achieved by mixing thoroughly throughout therefractory material which is to be i bonded into an integral article,normally by the presence,

of a binding agent, a magnetically responsive material, i.e., aparamagnetic material in a finely divided state. Typical of magneticallyresponsive materials are the transition metals of iron, nickel, cobalt,manganese, alloys thereof,-

certain compounds thereof, and mixtures. thereof. An iron powder forexample is an excellent example. This powder need not be ofanyparticular particle. size, but the larger the particles, the moreheat generated thereby.

In experiments, it was found that even iron and steel dust removed froma conventional machine shop grinder, for example, a Blanchard surfacegrinder, worked completely effectively. The novel composition is formedby mixing, preferably uniformly, throughout the particulate refractorymaterial, a small percentage of the magnetically responsive material.The paramagnetic powder must be in a sufficient amount to achieve arelatively rapid heat generation throughout the mixture when it isexposed to a high frequency magnetic field.

is not real critical. It may be present anywhere from about one percentup to twelve or more percent, depending upon the reaction time andamount of heat desired. A typical core sample formed of a silica sandmixed with six percent phenol formaldehyde bonding resin, and sixpercent iron powder, mixed uniformly throughout, cures in the magneticfield in approximately three seconds. In-

creasing the iron powder to about twelve percent lessens the time ofcuring below about two seconds. Percentages smaller than six percentlengthen the amount of timecure the particular. binder employed. Mostbinders can It is also intended, in its very,

It was found that the percentage of magnetically responsive substance becured'with a small percentage of magnetically responsive powder mixedthroughout the sand, by consuming more time.

When exposing a six to twelve percent magnetically responsive mixture ina magnetic field, the reaction occurs so-rapidly that if the mixture isinserted after the field is actuated, and at the rate of a few inchesper second, the reaction can be visually observed by the color change,as curing occurs. It is extremely rapid compared to conventionalmethods, i.e. about one-tenth to one-twentieth of that normallyrequired.

Preferably, it is best to apply the magnetic field simultaneously to theentire article to be cured, by first placing the mixture in the area andthen activating the magnetic field for a limited time interval. Thisachieves constant time application for all portions. The particular highfrequency of the magnetic field can be varied widely from about 800cycles from the generator type machine up into radio frequencies ofabout 450 kilocycles, depending upon the application. A typical magneticfield machine is a Reeves Induction Machine which normally operates fromabout 50 to 450 kilocycles. As is wellknown, these induction machinesare not limited to particular helical coil structures, but rather theprimary induction element can be formed, for example, of a copper tubeformed into the necessary configuration to induce a high frequency,secondary magnetic field in the sand. A flat pancake type structure canbe formed or even other configurations, to cooperate with the particularcore, mold or other article to be cured. Of course, efliciency is lowerif the typical helical coil is departed from, but the modifiedconfigurations are still effective.

By utilizing this method, the coil can be brought to the work ratherthan requiring the work to be brought to a heating source. Therefore,the activating magnetic field can be exerted in a variety of systemswhile allowing maximizing of system layout efficiency. This makes thecuring process a conveniently adaptable step in a manufacturing system,rather than a complex separate operation requiring vast amounts ofspecialized equipment and time loss.

It was found with testing that the retainer for the particulatesubstance for curing need not even be a heat conductive metal at all, asof course was previously necessary. In fact, even when a sheet paperpattern retaining the particulate refractory material with magneticallyresponsive powder mixed throughout, was inserted in a high frequencymagnetic field for a few seconds to cure the material, no harmful actionoccurred on the paper except for very slight increase of itstemperature. The pattern and substance could be held by a persons handdirectly in the field while curing occurred, without discomfort. Infact, this was the way in which the very slight temperature rise wasdetected. Therefore, the initial pattern for the cores, molds and otherarticles need not be formed by expensive procedures, out of heatconductive metals which are difiicult to pattern, but rather can beformed out of any support material such as plastic, wood, laminates ofvarious types, etc. Consequently, cores, molds, etc. can be baked andcured in a variety of ways, using a selected retainer of cardboard,paperboard, plastic, wood, etc. The pattern can be rapidly carved out ofwood, for example, on small production set ups. It can be massedproduced from plastic materials by molding or pressure formingtechniques. Consequently, this invention opens up a whole new area offorming procedures because it eliminates the present necessity ofexpensive core boxes, and enables the substitution of inexpensively andrapidly produceable patterns.

The amount of heat generated in the article is directly proportional tothe mass of that section. Every unit mass of the core receives the sameamount of internal heat generated at that particular local, to cause allparts to be locally heated the same amount, with no heating conductedfrom the surface inwardly. Further, when the paramagnetic material isuniformly distributed, the heat generated is in direct proportion to themass so that thick sections having internal portions do not becomeunderbaked and under-cured while surface portions become over-baked andover-cured, as frequently occurs with conventional techniques. 7

Because of these factors, shell molds can be made of varying thickness,and will still achieve completely uniform curing. Therefore, the backsurface of a shell mold can be made flat, for example, while the frontface is configurated accurately with the specific molding cavity patterndesired. Then, 'by stacking or placing the half shell molds intoface-to-face and back-to-back relationship, an entire series can beplaced together. Then, merely by supporting the ends of the series as byclamps, to keep them together, the backing material of sand or shotcustomarily used is no longer necessary for each pair of shell moldsections.

The magnetically responsive mate-rial can be readily incorporated into asand mixture by adding it during mulling, when the uncured phenolicresin binder is added. Since both are added as powders, they readily mixthroughout the mass in uniform quantities, as has been determined byexperimentation.

In fact, it was determined that the iron powder and phenolic resinpowder tend to mix so well with each other to coat each other, that thesmall iron particles cause local heating directly on the phenolic resinparticles to optimize curing action instantaneously over the entiremixture. In other words, the only substance that needs heating, thebinder, is heated since the paramagnetic powder clings to it. Therefore,it is not really necessary to heat the surrounding refractory particlesvery high at all, but just enough to be cohesive with the binder resinas it liquifies. It is no longer necessary to conduct heat through theparticulate refractory materiala difficult task at best. The heat iscreated and concentrated exactly where it is needed.

.The result is a uniform coating of the sand particles with the resinand powder to produce the optimum integral article. The resulting mold,core, or other article exhibits no disadvantages from the addedmaterials, but can be of better quality.

It will be realized that, if desired, the heat generated in each portionof an article can be controllably varied rather than the entire articlebeing completely uniformly heated as is usually desired with core andmold making. This variation is achieved simply by mixing a greateramount of magnetically responsive material in certain portions wheremore heat is to be generated. The heat generated in the magnetic fieldis greater in these portions, in proportion to the amount ofparamagnetic substance present. If it is desired to obtain a glazedsurface on a ceramic article, for example, by greater heating of thesurface portions than the interior portions, but still is desired to heat all interior portions uniformly, the surface is provided with agreater amount of magnetically responsive powder. The greater heatproduction at the surface causes increased temperatures and glazingduring the same curing time as the rest of the article requires foruniform curing.

A few illustrative specific examples of the method in composition are asfollows:

Example 1 A silica core sand containing 6% by weight phenol formaldehydebinder is mixed uniformly with 6% by weight iron powder. The mixture isplaced in a formed paper pattern and inserted by hand into a highfrequency 450-kilocycle magnetic field of a Reeves Induction Machine.The field is activated for approximately 3 seconds, The article isthereby cured, and is removed. It is slightly warm to the touch duringcuring. Very little vapor occurs. The article exhibits excellentstrength, and when broken, shows completely uniform curing throughout.

7 Example 2 A silica sand containing 3% by weight linseed oil is mixedwith 5% by weight iron .powder, packed into a wood pattern, and placedin a high frequency magnetic field of 200 kilocycles for 5 seconds tocure the binder.

Example 3 A fire clay :particulate refractory material is mixed with byweight melamine formaldehyde and 10% by weight iron powder, ,placed in aplastic pattern, and exposed to a high frequency magnetic field at 100kilocycles for 2 seconds.

Countless other examples could be recited, but would only undulylengthen this specification. The variations will be apparent to those inthe art upon studying the descriptive material presented. The inventionis intended, therefore, to be limited only by the appended claims andthe reasonable equivalents thereto.

I claim:

1. A method of forming an integral article from a refractory particulatematerial, comprising the steps of mixing uniformly throughout theparticulate refractory material a small percentage of binder and a smallpercentage of magnetically responsive material; forming the resultantmixture into a desired configuration; and subjecting the mixture to ahigh frequency magnetic field to heat and curethe binder.

2. A method of making cores and molds, comprising the steps of:providing a mixture of particulate refractory material having therein aheat responsive binding agent capable of binding said particulatematerial into an integral article when heated; dispersing through saidmaterial a small amount of a finely divided paramagnetic powder; formingthe resulting mixture into a desired configuration;

and applying to the resultant mixture for several seconds a highfrequency magnetic field, creating heat uniformly throughout saidmaterial in said binding agent in .proportion to the mass of themixture, to uniformly cure said binding agent.-

3. A method of forming a shell type moldassembly, comprising the stepsof: providing a refractory particulate material having throughout asmall percentage of heat curable resin binder and a small percentage ofmagnetically responsive material; forming the resultant mixture into amold section having a flat back face and a configurated front face;subjecting said mold section to a high frequency magnetic field for afew seconds, thereby creating uniform internal heat and curing thebinder; and place, ing a plurality of the cured mold sections intomutually supporting face-to-face and back-to-back relationship to form aplurality of mold cavities.

References Cited by the Examiner 819,393 9/1959 Great Britain.

1. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

2. A METHOD OF MAKING CORES AND MOLDS, COMPRISING THE STEPS OF: PROVIDING A MIXTURE OF PARTICULATE REFRACTORY MATERIAL HAVING THEREIN A HEAT RESPONSIVE BINDING AGENT CAPABLE OF BINDING SAID PARTICULATE MATERIAL INTO AN INTEGRAL ARTICLE WHEN HEATED; DISPERSING THROUGH SAID MATERIAL A SMALL AMOUNT OF A FINELY DIVIDED PARAMAGNETIC POWDER; FORMING THE RESULTING MIXTURE INTO A DESIRED CONFIGURATION; AND APPLYING TO THE RESULTANT MIXTURE FOR SEVERAL SECONDS A HIGH FREQUENCY MAGNETIC FIELD, CREATING HEAT UNIFORMLY THROUGHOUT SAID MATERIAL IN SAID BINDING AGENT IN PROPORTION TO THE MASS OF THE MIXTURE, TO UNIFORMLY CURE SAID BINDING AGENT. 